In all paper production, whether cardboard, board or tissue, at least one fiber web is conveyed continuously through a machine from the moment when the fiber web is formed in a wire machine until the finished web is wound up to form a reel in a reel-up. The machine consists of a plurality of different and separate machine parts, sections, arranged in sequence one after the other, that gradually transform the fiber web to a finished reel of paper. The various sections are essentially, wire, press and drying sections and a final reel-up.
Board and cardboard differ from what is generally termed paper primarily in the greater demands for rigidity and thickness. Cardboard also generally consists of several layers couched together, each layer having a different stock content. The same term is frequently used for several different types of paper material and the terms used are therefore to be considered as somewhat flexible. However, the following approximate limits should be considered as prevailing generally. Between paper and cardboard the limit lies at a basis weight of about 100 g/m.sup.2, whereas the limit between cardboard and board is sometimes stated to be about 600 g/m.sup.2. The problems that arise when feeding along a new material web in a machine are substantially the same, regardless of the type of paper, i.e., paper, cardboard or board, or of the machine design such as a paper making or board machine that is used. The term "paper" will be used in the following unless a particular type of paper or machine type is expressly intended.
When a new web end is to be passed through the separate sections of the papermaking machine, which has to be carried out after a web rupture or a machine stop, this is done by means of threading. In order to facilitate threading, a narrow strip is separated from the rest of the web by a cutter, usually in the form of a water jet. This separation is performed, for instance, on the wire immediately before the web is transferred to the press section. The leading end of the paper web is thus shaped to form a leader with a width of suitably about 100-200 mm. This leader is either blown over to the press section with an air cushion inside the wire or, in a closed transfer, is gripped by a suction roll so that the web is lifted over to the press section.
The leader is then fed on through the papermaking machine and over all the open draws existing therein, either, entirely by hand or with a certain amount of assistance from various blowers for the purpose. The term "open draw" refers to the gap formed between a first machine part and a second machine part following the first in the machine direction, over which gap the web is forced to run freely without support from a supporting surface such as a wire or felt, for instance. The blowers, at least in theory, force the carried leader in a certain direction so that it passes over the open draw and further on into the next drive section.
The simplest blowers are ordinary compressed air nozzles which have openings or small holes drilled with a certain mutual pitch, placing and number in the longitudinal extension of the nozzle. Since the ability of these more or less stationary blowers to transfer the end of the web in a correct manner is to a great extent dependent on the vibratory movements, web flutter, which exist in the entering leader, these blowers are far from sufficient to reliably and precisely transfer the web during threading.
The open draws may be situated between the wire section and the press section, for instance, as is the case in slow-running machines where the web is lifted off the wire at the couch roll and thus is passed over to the felt; inside the press section between one or more of the presses arranged therein; and between the press section and the drying section. The risk of web rupture increases substantially at the open draws and therefore one tries to replace these with a web transfer which is closed as far as possible. This can be achieved, for instance, by using a pick-up roll wrapped by a felt when transferring the web from the wire, or with connected press nip in the press section.
A drawback with connected press nips is that the two sides of the paper web, the wire side and the upper side, will not be the same since, contrary to the wire side, the upper side does not run against a smooth roll surface but all the time runs against felted rolls. If a web is desired having both sides as alike as possible, the upper side of the fiber web must also be allowed to run against a smooth roll. For this reason a special type of press section is usually used in this case which includes a separate third or fourth smooth press nip. However, this means that an open draw is unavoidable, with the associated transfer problems.
Another problem is the open draw between the press section of the paper machine and its drying section. Attempts have been made, however, to minimize the gap across which the web must run freely by arranging some form of web stabilizer to partially bridge the gap. Such a web stabilizer is usually constructed so that the drying wire is drawn in a loop around a guide roll arranged as close as possible to the outlet from the central roll of the press section, or the last press roll in the direction of the machine. At least one blow box is arranged inside the drying wire loop and helps to guide the leader with the aid of a flow of compressed air operating in the direction of movement of the paper web so that a vacuum effect occurs, a Coriolis effect, on the lower side of the web stabilizer bringing the leader against the surface of the drying wire loop. Once the leader has been taken over by the web stabilizer it is conveyed further by the drying wire loop up to and into the first nip formed at the first drying cylinder. The open draw between press section and drying section is thus greatly reduced, thereby facilitating also the transfer of the paper web. However, an open draw, although somewhat narrower, will still remain between the drying wire loop and the press section and therefore still constitutes a serious risk of unsuccessful threading or, in the worst case, a web rupture.
The guide roll, and also the drying wire running around it and enclosing the blow box, have a blocking effect on the air flows necessary to feed the leader forwards, the flows already having a very limited sphere of influence close to the drying wire loop. The suction effect that these blow boxes give rise to can therefore only grip the leader if it can be brought close enough and since before passing through the drying section, the leader still contains so much water that it is weighted down by moisture and very much still adhered to the press felt, threading at this open draw must still be performed by hand in spite of the web stabilizer. This can usually be done with the aid of a portable compressed-air device that the operator inserts in the space between press section and drying section. This manual procedure constitutes an extremely time-consuming and therefore expensive problem in the paper industry, particularly since developments in other respects tend towards higher speeds and lower basis weights in the running paper webs.
Pivotable blow tables are disposed further on in the paper machine, where the fiber web is considerably drier and therefore much lighter and not so firmly adhered. These are folded up from beneath, i.e., against the lower side of the fiber web. Remaining open draws can thus be bridged so that the web is transferred carried by an air cushion from the blow table, without any greater risk of it adhering. However, these blow tables require considerable space to move and obviously cannot be installed if a wire or felt intersects the path of movement it requires, as is the case in the open draws described above inside the press section and at the connection of the press section to the drying section. Since, when folded up, the pivotable blow table thus cuts the moving fiber web, such a blow table is also arranged, with the aid of a knife means, to cut off the leader. This is possible with a drier and thus more easily cut web, but as with a moist web in a press section it is almost impossible to achieve without the use of the water jet mentioned above.
Furthermore, the physical space in the press section does not allow for installation of the same type of equipment as is used in the drying section; the equipment, which functions satisfactorily when a drying web is moving, would result in the leader becoming irretrievably adhered when used for the fiber web in the press section which is still very wet.
Once threading has been successfully performed in a press section, the above-mentioned cutter is moved across the web so that the width of the leader increases until the web is again moving at full width. While threading is being performed the rest of the web is permitted to run down into a device called reject disintegrator, intended for disintegration of pieces cut or rejected from the paper web.
The development of the threading process described has been necessary in order to cope with the ever increasing speeds. Manipulating the leader by hand always entails risks and these increase with the speed. It will be readily understood that manual threading furthermore is extremely inefficient and expensive. It should therefore be obvious that it would be of extremely great significance to the paper and board manufacturing industry if an automatic and adjustable threading device could be developed which can be used quickly and safely to bridge the open draws still existing between press section and drying section, as well as the one in a press section comprising one or more separate press nips.
One object of the present invention is to at least for the most part remove the problems mentioned above and provide an improved device for leading and adjusting the position and direction of a leader when threading in a paper or board machine.
Another object of the invention is to provide a device that will enable fully automatic threading to be performed through all parts of a paper or board machine, all open draws thus being bridged in a safe and reliable manner, including the open draw between the press section and the drying section.
Yet another object of the invention is to provide a device for threading in a paper or board machine, in which the leader is transferred in a safe and reliable manner to a web stabilizer advanced from the drying section towards the press section.